1. Field of the Invention
The present invention relates to an image pickup apparatus, a control method for the same, and a program for implementing the control method, and in particular to an image pickup apparatus that designates a region or position on a screen of a picked up image and carries out a focus adjustment for the designated the region and so on, a control method for the same, and a program for implementing the control method.
2. Description of the Related Art
In the field of consumer image pickup apparatuses, such as video/still cameras (digital camcorders), various attempts have been made to simplify operation and obtain higher-quality images, and various techniques have been proposed. The auto-focus (AF) function equipped as standard on digital camcorders in recent years removes the trouble of adjusting the focus every time an image is shot, and is therefore a typical example of a function that attains the object of making it easy to obtain high-quality images.
At the same time, great progress has been being made in miniaturizing and reducing the weight of digital camcorders. One reason for such progress is the existence of inner-focus type lenses. Inner-focus type lenses are lenses which are capable of shooting a subject just close to the front surface of the lens by not mechanically linking a correction lens and a zooming lens with a cam and instead storing movement loci of the correction lens as lens cam data in advance in a microcomputer, driving the correction lens in accordance with this lens cam data, and performing focusing using the correction lens. Such inner focus type lenses have the advantages of low cost, simplification of the system, and a reduction in size and weight of the lens barrel.
FIG. 6 is a schematic diagram showing the construction of a conventionally used inner-focus type lens system.
In FIG. 6, the lens system includes a fixed first lens group 101, a second lens group 102 for zooming, a diaphragm 103, a fixed third lens group 104, a fourth lens group (hereinafter referred to as the “focus lens”) 105 that has a focus adjusting function and a so-called compensating function that compensates movement of the focal plane due to the zooming, and an image pickup surface 106.
In this lens system, since the focus lens 105 has both the focus adjusting function and the compensating function, even if the focal distance does not change, the position of the focus lens 105 for focusing on the image pickup surface 106 differs according to a distance to the subject. If the position of the focus lens 105 for focusing on the image pickup surface 106 is continuously plotted as the distance to the subject is changed at respective focal distances, a graph like that shown in FIG. 7 is obtained. During zooming, if one of the focus loci shown in FIG. 7 is selected according to the distance to the subject and the focus lens 105 is moved according to the selected locus, zooming can be carried out with no blurring.
However, according to the AF described above, an image pickup apparatus, such as a camera, determines by itself photographic conditions and adjusts the lens position to a state that is thought to be appropriate for the photographic conditions, so that there are cases where the photographer's wishes are not reflected in the image. For example, when an AF operation is carried out using information for the entire image pickup screen in the case where a distant subject and a proximate subject are both present in the image pickup screen, it is likely that one of the plurality of subjects will be put in focus, but the image pickup apparatus cannot determine which is the main subject on which the photographer wishes to focus.
To avoid such situations as much as possible, a method that measures the distance to a subject located on the center of the image pickup screen with priority and executes an AF operation based on the result of the measurement is generally used. This is based on the fact that photographers often place the main subject on the center of the screen during photography. However, there are cases where this method is not able to appropriately adjust the focus for the main subject when the main subject is positioned away from the center of the screen.
On the other hand, to set the optimal focus regardless of where the main subject is positioned on the image pickup screen, an image pickup apparatus that can select the main subject based on the visual axis of the photographer gazing with his eye through the finder has been proposed (see Japanese Laid-Open Patent Publication (Kokai) No. H04-154165). According to this image pickup apparatus, it is possible to freely change the position of the main subject while specifying a distance measuring region. The position designating means for selecting the main subject is not limited to the visual axis detecting means, and another position designating means, using a pointer member such as a joystick or a mouse, that synthesizes amounts of movements of the pointing member on two axes in accordance with the subject to determine the direction and the position in and to which the pointing member has moved and thereby determine the main subject can be envisaged. Further, a selection function that selects a distance measuring region as desired from a plurality of distance measuring regions set in advance using the visual axis detecting means or the position designating means can be envisaged as an easily realizable function.
In recent years, there have been increased functions having compatibility between digital camcorders and digital still cameras. On the one hand, moving image recording functions of digital still cameras are becoming increasingly advanced, while digital camcorders on the market that are capable of recording still images and moving images onto card-type storage media as well as onto tape-type storage media. For digital camcorders, an operation method where still images are recorded using a “photo” switch and moving images are recorded using a “trigger” switch (start/stop switch) is becoming increasingly common.
The above-described function of selecting one or a plurality of regions are selected out of a plurality of distance measuring regions should preferably be available anytime while the camera is in operation. However, the above function that reflects the photographer's wishes is a so-called “manually” operating function, and therefore such operations cannot be carried out without performing a predetermined key operation. For example, it has been necessary to carry out a three-step operation in which switching to a selection mode for selecting a distance measuring region is carried out in a first step, changing or switching the selected distance measuring region is carried out in a second step, and confirmation of the selected distance measuring region is carried out in a third step. The first step and the third step are carried out using a different switch to one used for the second step. It is difficult to carry out two types of switch operation while looking through the finder or at a monitor.
Further, as is clear from FIG. 7, referred to above, in a zooming operation of moving a zooming lens from a “wide-angle” position to a “telephoto” position, the focus lens moves from a region where focus loci converge to a region where the focus loci diverge, so that at the wide-angle position it is not possible to determine the subject distance of the locus that enables a focused state to be maintained from the wide-angle position to the telephoto position. For this reason, in general, zooming is carried out towards the “telephoto” end while determining a focus locus by maintaining the focused state using AF information. However, during high-speed zooming where only a reduced number of detection of AF information can be obtained, making it difficult to maintain and determine the focus locus. When a high-speed zooming operation and switching of the distance measuring region are carried out at the same time, that is, zooming is carried out while the distance to a main subject is changed, it is difficult to maintain the focused state, resulting in occurrence of blurring. Since time is required to focus on a new subject during change of the distance measuring region, problems such as significant blurring can occur, which prevent high quality images from being obtained.
During still image photography, a method for focusing by a digital camera has been generally employed, in which, to increase the reliability of focusing, the focus lens is moved within a predetermined search range by operating the photo switch to obtain a focusing signal indicative of focusing having been reached during the movement of the focus lens, and the position of the focus lens at which the focus signal assumes the maximum degree of focusing is detected by calculation or the like to find a focused position. However, if the distance measuring region is changed during the above focus search, the focus search operation searches for foci for different subjects during the search, resulting in the problem that proper focusing cannot be obtained. Also, when the distance measuring region is changed during the recording of moving images activated by operation of the trigger switch, blurring occurs until the apparatus responds to switching of the main subject, so that it has not been possible to obtain high-quality images.
Unlike still image photography, during moving image photography photographic effects and image lay-out are usually carried out by camera work and zooming, and therefore the stability of focus is very important for successful formation of an image. To this end, the stability of focus is given priority over focusing response for the main subject, so that it is desirable to automatically optimize the distance measuring region according to the photographic conditions, in various types of distance measurements such as “entire screen distance measurement”, “center-prioritized distance measurement”, and “periphery-prioritized distance measurement” for a hollow subject. However, during selection of the distance measuring region, it is unclear whether the photographer will next operate the photo switch or the trigger switch. If the selected distance measuring region is at the edge of the screen and moving image recording is carried out using the trigger switch, if distance measuring region control is automatically started in the center-prioritized distance measurement mode, for example, the recording of moving images starts before the focusing operation for a central subject due to the change of the subject is completed. This results in blurred images being recorded at the start of recording.